Relining of Urea Reactors at Plant Site

Urea reactors were damaged seriously and extensively due to the erosionlcorrosion oftheir internal lining. By shutting down the reactor for safety, major repairs were

carried out successfully with the existing equipment at the actual site.

Cesare Miola and Franco GranelliSnamprogetti, Milano, Italy

Jorge Pirela and Julio AriasPetroquimica de Venezuela (Pequiven), Tablazo, Venezuela

Purpose of the paperby Pequiven. The repair was studiedjointly by Pequiven and Snamprogetti.

The purpose of the paper is to illustrate arepair made to urea reactors that sufferedserious damage due to a wide and deeperosion/corrosion event on their internallining.It is very likely that this repair was thefirst of its kind in the high pressureequipment of a urea plant, especially sincethe damage affected an area of more thana few dozen of square meters of lining.The repair was done at plant site. Thetime required was slightly more thanrequired for the turnaround.The damage was so extensive that forsafety reasons the reactors could nolonger be operated. After the repair,however, the reactors were fully operativeagain in terms of reliability andsafety.The urea production, which was atrisk of failure for a long time, was saved.The reactors forming the subject of thispaper were installed in urea plants owned

Short profile of Pequiven andSnamprogetti

PEQUIVEN

The development of the petrochemicalindustry in Venezuela was promoted bythe Venezuelan government towards theend of the 1950's by establishingPETROQUIMICA DE VENEZUELAS.A.(Pequiven) - an affiliate ofPETROLEOS DE VENEZUELA S.A. -responsible for converting thehydrocarbons available in Venezuela topetrochemical products for both the homemarket and export.Pequiven operates three complexes:The JOSE ANTONIO ANZOATEGUIcomplex, now being developed, is locatedat José, in the east of the country. Thiscomplex produces methyl-ter-butyl-ether(MTBE) and methanol; these products are

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mainly intended for export.The MORON complex is located atMoron, in the center of the country: theoutput consists of fertilizers and NPKblends for the home market.The ZULIA complex is located in theTablazo in the west of the country: theoutput consists of olefins, plastics andfertilizers: the olefins and plastics are bothfor the home market and export, while thefertilizers (ammonia and urea) are mainlyfor export.Since the late 1980's the Pequivencomplexes have promoted theestablishment of fully private concernswhich utilize the raw materials thatPequiven and its associated companiesmake available.Since 1989 Pequiven has improved itsplant management policy through betterquality production processes, using anoverall production quality programincluding plant safety and environmentalissues.

Introduction of quality control into theprocesses by a specialist team working inclose cooperation with the technicaldepartments and plant operators has madeit possible to take action to cut costs andrecover equipment no longer consideredoperative: this is the case with the reactorsin the Tablazo urea plants, and constitutesthe subject of this paper.

SNAMPROGETTI

Snamprogetti, the internationalengineering contractor and technologycompany of Italian ENI Group, isengaged worldwide in the development,design and construction of industrialfacilities and associated infrastructuresthat include pipelines and plants foroffshore processing, refining, gastreatment, fertilizers and chemicals.With a background of over thirty-eightyears of professional experience,Snamprogetti is able to offer its customershighly qualified services and supportranging from individual packages ofintegrated services to complete turn-keyprojects.

It should also be remembered thatSnamprogetti provides assistance tocustomers with regard to corrosionproblems and general maintenance. Forcases where visual inspection is notsufficient Snamprogetti has qualifiedvarious organizations all over the world;in line with Snamprogetti indications andcustomer requirements they performspecial nondestructive tests.For special repairs Snamprogetti is inconstant contact with equipmentmanufacturers, who can make theirtechnicians and labour promptlyavailable.Well known internationally is theSnamprogetti urea technology, alreadyused in some 80 plants throughout theworld.

Brief notes on corrosion in ureaplants

Ever since the early development ofindustrial urea plants based on directreaction between ammonia and carbondioxide, urea plant designers and ownershave been faced with the problem ofcorrosion.This problem was serious for many years,since it not only endangered the goodperformance of the urea plants (safety, onstreamfactor, product quality) but alsoformed an obstacle to improve the processitself.Silver and lead have been largely used aslining materials to protect the surface ofsome apparatus.The situation has now considerablyimproved, but the process conditions,temperature, pressure and particularfluids composition in the variousoperating stages of the process (with thepresence of intermediate compounds) stillrequire not only the careful selection ofconstruction materials, but also adequatedesign. In addition, manufacture of theequipments, or parts thereof, must becarried out with great care and in linewith specific procedures issued by theProcess Licensor.

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Apart from material selection, it is thusimportant during the design stage toensure that under all circumstances thefluidynamics inside the equipment will beappropriate: both high velocity of fluidsand areas with no velocity (dead areas)are to be avoided.During workshop fabrication, care mustbe taken with details such as cuttingspeed, cooling solutions, heat treatmentand even the cleanliness of the workshopwhere the equipment is manufactured.For these reasons it is mandatory that atleast the equipment and machinery for thehigh pressure section must bemanufactured by Vendors qualified by theProcess Licensor. During normal plantoperation, as well as transitional periods,all procedures recommended by theProcess Licensor must be strictlyfollowed.Good maintenance is also an importantelement playing a fundamental role, notonly to increase the onstream factor, butalso to control good maintenance willcontribute to long plant life.One aspect of corrosion in urea plants hasto be absolutely clear: corrosion events,even though they have been studied for along time and in spite of the enormousamount of information obtained fromlaboratory tests and industrial plants, arenot yet fully understood and to someextent retard development of thetechnology.

Short description of the urea plants atthe Tablazo complex

Pequiven operates two urea plants (A andB) in the Tablazo complex, each with acapacity of 1,200 t/d.They are based on the total recycle C&IGirdler-Toyo Koatzu technology.Each plant has two urea reactors runningin parallel; there are thus four reactors,each with a capacity of 600 t/d.The two plants came into operation in1973.The process is based on the reactionbetween ammonia and carbon dioxide,

which are compressed to 230 bars andsent to the reactors, where two well-known reactions occur:

2NH3 + C02 = NH2 COONH4

NH2 COONH4 = NH2 CO NH2 +H20

The first reaction is exothermic andcompletely converts all the C02 to theintermediate compound: ammoniumcarbamate.The second reaction, the dehydration ofthe ammonium carbamate to urea andwater, is endothermic and is controlled byan equilibrium mainly governed bytemperature and by the ratios NH3/C02and H20/CO2 in the reactor. Theconversion efficiency of C02 to urea is67 per cent. Hence the aqueous solutionleaving the reactors contains urea,ammonium carbamate and the excessammonia.It is sent to a high, medium and lowpressure decomposition and recoverysystem with the aim of separating theammonium carbamate and excessammonia from the urea solution. Theammonium carbamate is recycled to thereactors, while the urea solution, (75%concentration) is conveyed to theevaporation section to be concentrated to99.7% urea by 2 steps under vacuum.The liquid urea at 138°C is sent to the topof the prilling tower where in free fall it iscrystallised and cooled in countercurrentair.

The prilled solid urea is then sent to thewarehouse by conveyor belt anddispatched in bulk or bagged.As stated ^above, there are four reactorsinstalled. The pressure resistant body ofeach reactor is of the multiwall type (3sheets each 47 mm - (1.85 in.) - thick)with a forged head and bottom and theyhave a lining made of urea grade AISI316L stainless steel sheets 17 mm (0.67in.) thick (see Figure 1).

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Description of erosion/corrosion foundin the reactor lining

In December 1989, in urea plant B,during the planned shutdown theinspection inside the reactors showed thatin one of them there waserosion/corrosion in the first 4 metres oflining starting from the bottom end.In August 1990 a similar inspectionshowed the same phenomenon also in bothurea plant A reactors. The damage wasalso similar in size.In all three reactors the erosion/corrosionof the lining was very severe andextensive, but nevertheless mainlyconcentrated in the first 4 metres starting,as mentioned before, from the bottom end.The average reduction in the thickness ofthe lining was 11 mm (0.43 in.), withminimal points of residual thickness closeto zero (see Figure 2).It had also been very rapid, as it hadoccurred in the 18 months between theprevious inspection and the present one.The appearance of the damaged liningwas that typical of an erosion/corrosionattack on modified AISI 316L in a ureacarbamate environment, and hencegeneralised erosion/corrosioncharacterized by strong intergranularattack.The appearance of the surface showedmainly oval, rectangular and roundcavities, as can be seen from photographsA, B and C attached.The welds likewise showed corrosiveattacks, which were particularly severe inthe first 4 metres from the bottom. Theinspections also revealed that the bottomdistributor was completely out of place, tosuch an extent that it was unable toperform its distributing function properly.The distributor anchoring bolts were nolonger efficient because they were nearlydestroyed by corrosion, something that isnot however new for urea reactors.The damaged sheets had a rough, darksilver appearance, while the upper sheets,not affected by the damage, were similar

in colour, but definitely in bettercondition.

Causes of damage and examination ofthe possibility of repair of the reactors

The serious situation evidenced by theinspections induced Pequiven to contactSnamprogetti. Excellent relations hadalready existed between the twoCompanies for a long time, in particulardue to the fact that Pequiven at the Moroncomplex had for years operated a 750 t/durea plant based on Snamprogettitechnology and there is still aSnamprogetti assistance contract inexistence.The problems posed by Pequiven relatedto examining whether it would be possibleto repair the reactors and return them toconditions of full operating reliability andsafety and establishing the causes of theabove-described phenomenon.It proved to be more difficult to solve thesecond problem than the first one.To determine the causes of thephenomenon the material of the sheetswas analysed and found, correctly, to beAISI 316L urea grade.The operating data were examined inorder to ascertain any absence ofpassivation oxygen, continuouslyintroduced into the reactors in the form ofair, or any anomalous temperatureincreases, or lastly any incorrectNH2/C02 and H2Û/C02 ratios.The fact that all these .parametersapparently were as they should be led tothe conclusion that one of the probablecauses of the phenomenon was to beattributed to excessive turbulence of thefluids in the bottom part of the reactors,originating from the failure of the bottomdistributor to function properly. Duringthe repair no modification was made inthe original design of the bottomdistributor.We recognise that this reason is not fullysatisfactory, although all urea plantexperts are well aware that excessivevelocity of fluids may produce erosionthat in whole or in part destroys the

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protective oxide layer, thus opening upthe way to the subsequent corrosive actionof the fluids on the material. Having saidthis, we nevertheless revert to what wassaid in paragraph 3 with regard to the notalways straightforward interpretation ofthese phenomena in urea plants. In orderto improve the situation in respect to apossible new phenomenon oferosion/corrosion, during the repair, asdescribe below, the material of the newlining as well as the material of thedistributor, including its supports andbolts, was changed from A1SI 316 L to25/22/2 Cr.Ni.Mo. Furthermore thepassivation system, consisting of airintroduced into the C(>2, was made morecertain in order to have a constant valueof oxygen.To resolve the first problem (possibility ofrepairing the reactors) a group wasformed between Pequiven andSnamprogetti. We should say right fromthe start that the group work led to apositive answer to Pequiven's problem: inother words it proved possible to recoverthe reactors and return them to operation.Technical-economic conditionsrelating to repair

Snamprogetti, after carefully evaluatingthe damage suffered by the lining of thereactors, made an in-depth study of thepossible repair procedure aimed atmaximum reliability and minimumexecution time.The Snamprogetti evaluations weresubmitted to Pequiven, which, after acareful examination, considered themeffective.The repair, also including completerestoration of the reactor, was thereforeplanned in the following actions:

A. Internal covering with new lining ofabout 4 metres of the reactor, andcovering of all circumferential weldsof the old lining with 100 mm (4 in.)wide strips.

B. Removal of all the old plate supportsand installation of new ones.

C. Grinding down to sound material andrewelding of all remaining welds incontact with the process fluid (i.e. thewelds of manhole, nozzles, distributorsupports, etc.).

In connection with the new lining and thecovering strip over the circumferentialwelds, a system of weep holes wasinstalled (see Figures 3 and 4) capable ofimmediately signalling any leakages.After the technical aspect of the repairhad been defined, consideration was givento the economic factors that would justifyit.The economic advantage was immediatelyevident by virtue of the fact that not onlydid the repair cost appreciably less thanthe cost of new reactors (on completion ofthe work it was in fact found to be abouta third), but it also saved plantproduction, which would otherwise havebeen lost for at least 12 months - the timethat would have been necessary forprocurement of new reactors.There was no doubt that the economicbalance was decidedly in favour of repair,which above all was carried out in quite arestricted period of time.

Stages preliminary to repair

Before repair was started, the followingguidelines and procedures wereestablished:

It was pointed out that the repairwould be carried out withSnamprogetti project and technicalassistance, in line with theSnamprogetti specifications relatingto high pressure urea equipment andmaterials.

- For performance of the repair theselected company was NUOVOPIGNONE of Florence (Italy), one ofthe companies qualified bySnamprogetti for construction of highpressure urea equipment.

- Before the start of the repair, allmaterials used (steel sheets andwelding materials) were subjected tothe chemical analyses and corrosiontests envisaged by the Snamprogetti

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specifications, and the necessarychecks were made by Pequiven on thequalifications of the welders to beused for the repair work.All necessary safety devices wereprepared and agreed upon withPequiven in order to perform the workunder orderly and safe conditions.To guarantee job execution within thescheduled time, an inspectionprogram was prepared to assure thequality of the repair and to avoidrework.

Description of repair carried out

Before anything else a nondestructive testwas made on all inspectionable welds ofthe pressure resistant body.Tests were then made on the mostseverely eroded/corroded parts of thelining (see Figure 2 and Picture D) inorder to make sure that any carbamatelosses had not damaged the pressureresistant body.After it was certain that the pressure bodywas in good condition, the repair wascarried out as follows:

A.) Complete covering of about 4metres of lining and allcircumferential welds-

1) The surface of theeroded/corroded lining wasrendered reasonably smoothby eliminating the mostconspicuous depressions.This was done by filling thecavities with urea grade fillermaterial in the less extensivezones, whereas the extensivecavities were filled withCr.Ni.Mo. 25/22/2 stripmaterial. This strip wasfastened to the lining by tackwelds. A reasonable smoothsurface was of coursesubsequently obtained bygrinding.

B.)

C.)

2) Testing was then carried outwith dye penetrant on all theabove-mentioned welds andfillers, to evidence anydefects, if any.

3) New weep holes were madeby inserting AISI 316 tubesand welding them to thedamaged lining'(see Figures 3and 4).

4) The eroded/corroded liningwas then coated withCr.Ni.Mo. 25/22/2 material 5mm (0.2 in.) thick, inaccordance with theSnamprogetti specification(see Figure 3 and 5).Obviously the courses werepreformed with dimensions topass through the manhole.The courses were carefullyinstalled, with special tools,on the existing lining andthen welded to it with the useof weld material prescribedby the Snamprogettispecification.

Removal of all existingtray supports andinstallation of new ones

All the old tray supports wereremoved, after which a carefulinspection of the lining was madeby means of dye penetrant. Thenew supports made of Cr.Ni.Mo.25/22/2 material were theninstalled and welded to the lining.These new welds were tested withdye penetrant.

Grinding down to soundmaterial and recharging of allwelds in contact with theprocess fluid

All lining welds and all parts

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welded to the lining in contactwith the process fluid wereground down to the soundmaterial. After the grindingoperation, tests were made withdye penetrant. The welds werethen overlayed with weld materialin accordance with Snamprogettispecifications.After this operation a furthercheck with dye penetrant was ofcourse made.

Final testing

The final checks on the repair were asfollows:

• hydraulic test at the pressure specifiedby the project standards

• check of ferrite content according tothe Snamprogetti specification

• leak test on the welds in the newlining according to Snamprogettispecification, and leak test withammonia, as requested by Pequiven

• dye penetrant test

• final visual inspection, with particularreference to the requirements of theSnamprogetti specificationsconcerning weld appearance, ironpollution, and the drying of surfaces.

Time taken to carry out repair,and operating results

perfect. In any case, during these twoyears (and in the following ones down tothe date of this paper and hence inpractice four years after repair of the firstreactor) there have been nocorrosion/erosion or other inconveniences.This demonstrates that repair as studied isnot only possible, but was actually carriedout with efficient workmanship.

Conclusions

The above description demonstrates thatthese large repairs to high pressureequipment of urea plants can be carriedout with the equipment still in the plantwhere they are installed, in timesequivalent to a turnaround, and on alldifferent types of equipment and withdifferent materials, at the same timeguaranteeing safety and productioncontinuity.The success of the repair carried out onthe three damaged reactors at Pequiven'sTablazo complex was possible largelybecause of the close cooperation that thePequiven personnel gave in the field ofsafety and the equipment made available,as well as the experience and skillnessshown by the Pequiven inspectiondepartment's personnel, which arrangedfor the proper checks and tests to becarried out during and at the end of therepair.

Repair of the three reactors was carriedout at different times. On average 20 dayswere required for repair of each reactor,or in other words a time equivalent to anormal turnaround. Hence there waspractically no loss of production.The results of repair were excellent.Two years after start-up of the repairedreactors they were carefully inspected,and the conditions were found to be

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DISCUSSION

G.R. Prescott, Consultant: I have asked this ques-tion of every other licensor expert in the world thatI have encountered. I have been involved with anumber of repairs on urea reactors, all lined with316L. We've known for at least 20 years that25/22/2 Cr.Ni.Mo. is infinitely more resistant tothis process than 316L, and I don't understand whythe licensors do not specify it in the original fabri-cations. Can you tell me why?Miola: As I said in the beginning, this plant wasput in operation in 1973 when the 25/22/2Cr.Ni.Mo. material was not available.Prescott: Do you specify it in your newer plants?Miola: No. In our new plants we don't specify25/22/2. We specify AISI316L urea grade.Prescott: That's what I don't understand.Miola: According to our specifications, we have aspecial prescription for this material.Prescott: The increase in cost of the final finishedvessel would be minimal if you specified the25/22/2.Miola: That's right.Prescott: Knowing that it's much more resistant tothe urea process fluid, I would think you wouldconsider specifying it for newer plants. I still don'tunderstand why the licensors don't do that.Miola: Often there is an international bid and thecosts of a complete plant are involved. An amountof US $70,000 or less may be sufficient to get anorder for a new plant. Anyway according to ourspecifications, in our process AISI 316L materialurea grade has proved sufficient to resist. We havereactors still running for 20-25 years without anyproblems.W.A.G. Lemmen, Stamicarbon BV: I heard Mr.Miola say that these reactors were not ofSnamprogetti design. To complete the story, I wantto add that these reactors were also not ofStamicarbon design.Miola: Thank you very much. I wanted to providethe real history of the reactor, not focus onSnamprogetti or any other company.R.C. Frey, M.W. Kellogg: Did you say that thecausative factor was a distributor failure? I thoughtturbulence and erosion was the cause for the liningerosion that you showed and that the distributor

was damaged in the reactor.Miola: Yes.Frey: Was this a causative factor in your liningproblem? I want to know what you did in terms ofrepairing that distributor or what changes hi designyou may have made.Miola: Here is the distributor, where you have CC>2inlet, recycle, and the ammonia inlet. This distribu-tor was completely out of place. The bolts werecompletely corroded, and so its original functionwas not made properly. We input one of the causesof that problem due to the fact that the distributorwas not properly functioning; however, we are notsure about that. In any case, we have replaced thedistributor with the same design but in a moreresistant 25/22/2 Cr.Ni.Mo material.A.M. Al-Jabr, Saudi Arabian Fertilizer. Did youmention anything about oxygen level, ppm's, forpassivation of the reactor?Miola: Yes, we have checked all these parametersbased on the data available in the plant controlroom and the result was that these parameters werein accordance with the process prescription.Al-Jabr: How much normally do you keep there inthe specification as per the process description?Miola: Sorry, I am not able to give an answerbecause this is not our process.Al-Jabr: Referring to Figure 5, after the job andthe next inspection was completed, did you findany deformation in the liner itself? As you can seehere, the old liner is not smooth enough. I wasexpecting there would be deformation of the newliner with that high-pressure reactor in that condi-tion.Miola: As you can see, as I mentioned in the paper,at this stage we have filled up all these cavities bywelding where they were small, and by a stripoverlay where the depth was remarkable. After thatthe surface was ground in order to obtain areasonably smooth surface.Al-Jabr: So at the next inspection you did not findany deformation there, at all?Miola: No, because you have an advantage withstainless steel as its elongation is higher than forcarbon steel. Even with some gaps, the lining willadapt to the original carbon steel body when the

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equipment is in operation.J. Pirela, Pequiven, SA: l want to make clear thatduring the last two inspections we haven't seen anydeformation or cracking in the lining. The inspec-tions have been extensive, so the lining seems to bein place and in good shape. Commenting on Mr.Prescott's remark about the material we used forthis service, we used 25/22/2 in a new reactor thatwe put in service. This material was also used forthe repair of the lining of three reactors and thecladding of a decomposer. So, as a standard we areusing 25/22/2 in urea/carbamate service.G. Collis, Simplot Canada Ltd.: Could you pleasetell me whether you did any ammonia testing onthe weep holes prior to and again after the repairwas done, and if so, what levels did you find?Miola: In Snamprogetti specifications we usuallyprescribe a shop helium test, but on-site, we preferan ammonia test, as we did for these repairs. Afterthe complete repair of the reactor, we performed anammonia test. We introduced ammonia through theweep hole, and we checked all welds with areagent. There are two methods. One reagent is apaste, and the other is ammonia sensitive paper. Inmy opinion the most sensitive is the paper fordetecting leaks in high-pressure equipment.Collis: Do I understand that you put pressure fromthe outside, from the weep holes?Miola: Yes, a little pressure. Just a little ammoniathrough the weep hole.Collis: Finally, when the unit is in operation hasthere been any ammonia or other process gasescoming through the weep holes?Miola: When the plant is in operation and youhave a leak due to the differential pressure of 150bar, you can see vapor or a continuous liquid flow

coming from the weep hole. Normally in the plantoperators go around the high-pressure equipmentevery 12 hours, and if there's a leak they can detectit. Another possibility, which we don't recommendbecause we think it's dangerous, is to connect allthe weep holes with proper tubes and circulatenitrogen through the tubes. In our opinion this con-trol method is dangerous, as it can cause mistakes.Collis: Would I be correct in assuming that therewould be no way to tell if there were a few partsper million of ammonia coming out of the weephole during normal operation?Miola: No. One possibility is to detect leakagesthrough the weep hole using Drager tubes. Thisway you can detect even a few ppm of ammonialeaks. Normally, we prescribe inspecting the weephole each three months. All the weep holes must beflushed with steam in order to make sure that theyare free from dirt. In the event of a carbamate leak,it can flow out freely.Pirela: I want to point out what our company hasdone. Every three to six months we put on scaf-folding and test that the holes are open. We use 15psi air to do this.G. Peterson, Saskferco Products Inc.: Do you indi-cate in your presentation that three reactors wererepaired in this manner?Miola: Yes.Peterson: Did the other two reactors fail in a simi-lar manner as this one, and was it a distributionheader, as well?Miola: Yes, and it was the distribution header thatfailed in the other two.Peterson: They failed because of corrosion?Miola: Yes, the bolts were practically destroyed bycorrosion.

Franco Granelli Cesare Miola

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FORGING

LINING - HEAVY/ERODED/CORRODED AREA

SEE A, B, C,DETAILED PICTURES

BOTTOM DISTRIBUTOR

CO,INtET ¥ T ¥ NW3im.ET

RECYCLE SOLUTION INLET

MULTIWALL RESISTANT BODY

FORGING

OLD ERODED/CORRODEDUNING

ERODED/CORRODEDPORTION FILLED-UP BYWELDING AND STRIP, ANDFINALLY SMOOTHED

NEW LINING(0.2") 5 MM. THK.. IN25/22/2 Cr.Ni.Mo. MATERIAL

Figure 3. Reactor-eroded/corroded liningrepair.

Figure 1. Urea Reactor. MULTIWALL RESISTANT BODY

CARBON STEEL RESISTANT BODY

(0.67-) 17 MM. ORIGINAL LINING THICKNESS

(0.43") 11 MM. AVERAGE CORROSION LOSS

Figure 2. Reactor eroded/corroded iiningdetail.

Figure 4. Reactor repair by covering withstrip the old corrodedcircumferential weld.

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UNING SECTORS CONNECTING PADS

CARBOMSTEEL NEW LINING

IN SECTORS

OLDERODED/CORRODED,-— UNING

5.

SCALE 1:1

phenomena.

SCALE 1:1

Photo B. Reactor lining erosion/corrosionphenomena.

SCALE 1:1

c.phenomena.

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SCALE 1:2

D. window on cthe eroded/corrodedpoints of the lining In order t©cheek the Integrity ©I the reactorresistant body.